1. Technical Field
The present invention relates to a technology for manufacturing a microstructure, in particular, a structure (nanostructure) on the order of nanometers, using laser interference exposure. This technology is applicable to processes of manufacturing various nanostructures such as polarization beam splitters, phase retarders, and antireflective surfaces.
2. Related Art
If light enters a structure (subwavelength structure) on the order smaller than a wavelength (order of 380 to 780 nm) of visible light, a phenomenon such as polarization separation, birefringence, antireflection, or plasmon propagation occurs. As one of the methods for manufacturing a subwavelength structure, exposure technologies using laser interference are known (for example, see JP-A-2006-093644). In the exposure technology described in JP-A-2006-093644, a photosensitive film is exposed to interference light (light having interference fringes) obtained by crossing two laser beams. In this case, the period of a pattern formed by the exposure to the interference light depends on a wavelength λ of the laser beams and the cross angle between the laser beams. In theory, the shortest possible period that is realized by interference exposure is equal to λ/2. For example, if the wavelength is 266 nm, the shortest possible period of a formed pattern is 133 nm in theory.
In order to realize a pattern with a period shorter than 133 nm using the above-described interference exposure, the wavelength of the laser beams must be shorter than 133 nm. However, it is difficult to reduce the wavelength of the laser beams under the current circumstances. One of the reasons is that there is no source of highly coherent lasers with a wavelength sufficiently shorter than 266 nm. Among excimer lasers used in the fields of semiconductor lithography and the like are ones with a wavelength of 193 nm, 157 nm, or the like. However, such excimer lasers do not have sufficiently high coherence in the present time. Therefore, it is difficult to form a sharp latent image pattern using these excimer lasers. Another reason is that if the wavelength is shorter than 200 nm, it is difficult to perform exposure in an atmosphere. It is also difficult to obtain materials suitable for optical components (lens, mirror, etc.) included in a laser interference exposure system. This is because, for example, the absorption edge of quartz glass lies around 180 nm. Therefore, a technology is desired that forms a sharp latent image pattern and realizes a fine pattern using this pattern without excessively pursuing a laser beam of a shorter wavelength.